Solid state lighting devices are used for a number of lighting applications. For example, solid state lighting panels including arrays of solid state lighting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. A solid state lighting device may include, for example, a packaged light emitting device (LED) including one or more light emitting diode chips. Inorganic LEDs typically include semiconductor layers forming p-n junctions. Organic LEDs (OLEDs), which include organic light emission layers, are another type of solid state light emitting device. Typically, a solid state light emitting device generates light through the recombination of electronic carriers, i.e. electrons and holes, in a light emitting layer or region. LED chips, or dice, can be mounted in many different ways for many different applications. For example, an LED chip can be mounted on a header and enclosed by an encapsulant for protection, wavelength conversion, focusing, dispersion/scattering, etc. LED chips can also be mounted directly to a submount, such as a PCB, and can be coated directly with a phosphor, such as by electrophoresis or other techniques. Accordingly, as used herein, the term “light emitting diode” or “LED” can refer to an LED chip, including an LED chip coated or otherwise provided with phosphor, or to a packaged device, such as a packaged device that includes an LED chip and that provides electrical contacts, primary optics, heat dissipation, and/or other functional features for the LED chip.
Solid state lighting panels are commonly used as backlights for small liquid crystal display (LCD) display screens, such as LCD display screens used in portable electronic devices. In addition, there has been increased interest in the use of solid state lighting panels for general illumination, such as indoor or outdoor lighting. For such applications, it is generally desired for the lighting device to generate white light having a high color rendering index (CRI), so that objects illuminated by the lighting device will appear to have more natural coloring. In contrast, light that has a low CRI may cause illuminated objects to have a washed out or unnatural appearance.
For larger illumination applications, multiple solid state lighting panels may be connected together, for example, in a one or two dimensional array, to form a lighting system. Unfortunately, however, the hue of white light generated by the lighting system may vary from panel to panel, and/or even from lighting device to lighting device. Such variations may result from a number of factors, including variations of intensity of emission from different LEDs, and/or variations in placement of LEDs in a lighting device and/or on a panel. Accordingly, in order to construct a multi-panel lighting system that produces a consistent hue of white light from panel to panel, it may be desirable to measure the hue and saturation, or chromaticity, of light generated by a large number of panels, and to select a subset of panels having a relatively close chromaticity for use in the multi-panel lighting system. This may result in decreased yields and/or increased inventory costs for a manufacturing process.
Moreover, even if a solid state lighting panel has a consistent, desired hue of light when it is first manufactured, the hue and/or brightness of solid state devices within the panel may vary non-uniformly over time and/or as a result of temperature variations, which may cause the overall color point of a lighting panel made up of the panels to change over time and/or may result in non-uniformity of color across the lighting panel. In addition, a user may wish to change the light output characteristics of a lighting panel in order to provide a desired hue and/or brightness level of the lighting panel.
Solid state lighting sources may have a number of advantages over conventional lighting sources for general illumination. For example, a conventional incandescent spotlight may include a 150 watt lamp projecting light from a 30 square inch aperture. Thus, the source may dissipate about 5 watts of power per square inch. Such sources may have a luminous efficacy of no more than about 10 lumens per watt, which means that in terms of ability to generate light in a given area, such a source may generate about 50 lumens per square inch in a relatively small space.
A conventional incandescent spotlight provides a relatively bright, highly directed source of light. However, an incandescent spotlight may illuminate only a small area. Thus, even though an incandescent spotlight has a relatively high light output, it may not be suitable for general illumination, for example illumination of a room. Thus, when used indoors, spotlights are typically reserved for accent or fill-in lighting applications.
Fluorescent light bulbs, on the other hand, produce light in a manner that is more suitable for general illumination. Fluorescent light bulbs approximate line sources of light, for which the illuminance falls off in proportion to 1/r near the source, where r is the distance from the source. Furthermore, fluorescent light sources are typically grouped in a panel to approximate a plane source of light, which may be more useful for general illumination and/or other purposes, since the intensity of the light generated by a plane source may not drop off as quickly near the source as the intensity of a point or line source of light does.
The distributed nature of a fluorescent light panel and its suitability for illumination has made fluorescent light panels a popular choice for general lighting applications. However, fluorescent light may appear slightly bluish and/or may have poor color rendering characteristics. Furthermore, fluorescent light bulbs may present environmental difficulties, since they may include mercury as a component.